Currently, mainstream digital transmitters are classified into a digital cartesian transmitter (DCT) and a digital polar transmitter (DPT), and both types of digital transmitters include a digital modulator capable of modulating a digital baseband signal to a radio-frequency signal.
An existing digital modulator usually modulates a digital baseband signal (BB) and a local-frequency signal (LO) by using a logic gate (AND gate). As shown in FIG. 1, in order to ensure that a modulated radio-frequency signal RF_data achieves an ideal waveform, phases of a digital baseband signal BB and a local-frequency signal (LO) need to be well controlled before the digital baseband signal BB and the local-frequency signal LO enter the logic gate. After edges of the digital baseband signal BB and the local-frequency signal LO are aligned, a logical AND operation is performed, so that the modulated radio-frequency signal RF_data may achieve the ideal waveform.
However, in fact, in a case in which a frequency of the local-frequency signal LO and a data rate of the digital baseband signal BB reach up to GHz (G samples/s), and in a case in which there are voltage, temperature, and technique deviations, it is very difficult to align the edges of the digital baseband signal BB and the local-frequency signal LO. As shown in FIG. 2, the logical AND operation is performed in a case in which the edges of the digital baseband signal BB and the local-frequency signal LO are not aligned, and consequently, the modulated radio-frequency signal RF_data may generate an unnecessary high-frequency harmonic component, which causes phase noise that comes from the digital baseband signal BB.